A method of the kind in question is known, for instance, from DE-A-38 20 862. During recent years photothermal examination of the surface of workpieces has proved to be a successful method of investigating workpiece surfaces in a contact-free and non-destructive manner. With this method, excitation radiation modulated in intensity, especially laser radiation is applied to heat the surface of a workpiece at a point to be measured, and the heat radiated from the point to be measured is detected and analyzed.
It is known from EP-A-0 609 193 to direct a continuous or a modulated laser beam, modulated with mechanical interruption, against a surface to be examined. On that surface, the laser beam generates periodic heating which is detected by an infrared sensor, then converted, and applied to a computer for evaluation.
In the case of another known apparatus for photothermal examination the laser excitation beam and the thermal beam it generates at the point to be measured are collinear (see DE 43 43 076 A1).
Publication DE-A-38 20 862 discloses a measuring system provided with an aperture diaphragm to subdivide a pulsating laser beam into a plurality of partial beams which are directed simultaneously to a body to be examined. The heat radiation emitted by the body under examination is sensed by a thermo-image camera.
An apparatus for optical examination of workpiece surfaces, comprising a detector diode array on which illuminated points of a sample are imaged is known from DE-A-30 37 983.